MUSINGS ON SCIENCE & SOCIETY

TO QUIVER OR TO SHIVER !!

The title of this post – To Quiver, or to Shiver is taken from a new article published this month in Proceedings of Royal Society London B. The paper deals with an interesting question – If a organism is good at one thing, does it mean that its good at all others too? OR Do you have to pay a price for being the best at something?

What is it all about?

Parasemia plantaginis coll. Mus. Zool. Oulu (Photo credit: Wikipedia)

Wood Tiger Moths, a species of moth(Parasemia plantaginis) found throughout Europe are widely known for their different wing colourations depending on the place they live.

Now why would this moth have bright colours?

Wouldn’t it attract more predators and hence get eaten up?

It turns out colours are used by various organisms as a predator-defence strategy. There can be two kinds of colouration schemes:

Camouflage:It is one colouration-based predator defence strategy and benefits from variable coloration, which prevents predators from developing a search image for the most abundant phenotype of prey.Because, if the prey have a consistent camouflage colouration then the predator would after a few trials and errors, learn to identify the prey easily and then the prey would be wiped out due to the predation pressure.

Aposematism: Situated at the opposite end of the predator aversion strategy spectrum, where prey use conspicuous warning signals to advertise their secondary defences. Warning signals of such organisms are not expected to vary much because predators should learn to avoid a uniform signal more efficiently than a variable signal, leading to greater survival.

Now, Wood Tiger Moth the species in question as seen in the picture above show aposematism and use these conspicuous colours as a warning signal towards predators. But, contrary to the predictions for this warning signal homogeneity, many such moths possess variable warning signals.

So, if uniform colouration is so important for predator defence, then why does the Wood Tiger Moth show variation in colours?

One reason could be that though different hindwing colours (white or yellow) ,in case of Wood Tiger Moth form the basis of predator defence but as one goes towards the Northern Latitudes, melanization or a general blackening of the hindwing is necessary for maximal absorption of sunlight or thermoregulation.

The general blackening to which i refer is the deposition of melanin, a process called melanization. It is used by many organisms including us !! Its benefits include immune response, thermoregulation etc.

To give you an example, see the below picture of the samples which the authors took from Europe.

So, in general as one can see in the Alpine regions where the sunlight is very rare for the whole year, the moths have less than 20% colouration !! (see, picture above (b))

But why??

It seems that higher melanization limits the amount of other pigments important in the warning signal, thereby setting the stage for atrade-off between defensive signalling and thermoregulation.

The question which the authors looked upon is something like this:

Do variation in male hindwing melanization is linked to thermoregulation and/or involved in a trade-off with protective benefits of colour ?

What did they find?

The authors started off with a combination of field based studies and predation experiments with artificial moths. Have a look at them:

The amount of melanization increased with increasing latitude in Wood Tiger Moth males from Estonia to north Finland.

Individuals in the Alps had a significantly greater amount of melanin covering their hindwings than those in central Finland.

Melanization also varied more among individuals in the Alpine region compared with central Finland.

Greater melanization also increased the average rate for warming up for the moths.

Every 10 per cent increase in the amount of melanin resulted in a 16 per cent increase in the odds of being attacked.

Taken together,the results show the likely existence of a trade-off between thermoregulation and predation riskwith respect to the amount of melanin present on the hindwing in Wood Tiger Moths. Specifically, what they found was that greater amounts of melanin in both white and yellow males resulted not only in an increased ability to absorb radiation, but also increased the likelihood of attack by avian predators. The results provide evidence that the differences in costs and benefits of melanin in the two locations can drive phenotypic differences in the warning signal of males on broad and local geographical scales.

Take home message:

If an organism is good at having X, then it must come at some cost. And as X is very costly it does not mean that its not a successful strategy, if the benefits of the costly strategy(X) outweighs the high cost !!